The discovery of antimicrobial agents possessing unique structural motifs or a novel mechanism of action is critical to counter and control the rising incidence of drug-resistant pathogens (Hawkey et al., J. Antimicrob. Chemother. 2009, 64, i3-i10; Gould, Int. J. Antimicrob. Agents 2008, 32, S2-S9; Pfaller et al., Clin. Microbiol. Rev. 2007, 20, 133-163; Sanglard et al., Lancet Infect. Dis. 2002, 2, 73-85). Chemosensitization of resistant organisms is a complementary approach that capitalizes upon the existing arsenal of antimicrobials to combat this medical dilemma (Lavigne et al., J. Antimicrob. Chemother. 2010, 65, 799-801; Gallo et al., Int. J. Antimicrob. Agents 2003, 22, 270-273; Kim et al., Biochem. Biophys. Res. Commun. 2008, 372, 266-271; Cernicka et al., Int. J. Antimicrob. Agents 2007, 29, 170-178). By undermining the target pathogen's resistance mechanisms, it is possible to restore efficacy to previously ineffective drugs thereby prolonging their status as frontline treatments. This, in turn, affords critical lead time towards the development of novel antimicrobial drugs.
Fungi are a prominent cause of hospital-acquired infections that are becoming increasingly difficult to control (Pfaller, et al., Clin. Microbiol. Rev. 2007, 20, 133-163). Several compounds have been previously identified as chemosensitizers, increasing the susceptibility of various strains of pathogenic fungus Candida albicans (C. albicans) to fluconazole (FIG. 1) treatment (DiGirolamo et al., “Reversal of fluconazole resistance by sulfated sterols from the marine sponge Topsentia sp.” J. Nat. Prod. 2009, 72(8):1524-28; Cernicka et al., “Chemosensitisation of drug-resistant and drug-sensitive yeast cells to antifungals.” Int. J. Antimicrobial Agents 2007, 29(2):170-8; Gamarra et al., “Mechanism of the synergistic effect of amiodarone and fluconazole in C. albicans. Antimicrob Agents.” Chemother. 2010, 54(5):1753-61; Guo et al., “Plagiochin E, a botanic-derived phenolic compound, reverses fungal resistance to fluconazole relating to the efflux pump.” J. Appl. Microbio. 2008, 104(3):831-38). Cernicka et al. previously reported that the compound 7-chlorotetrazolo[5,1-c]benzo[1,2,4]triazine (CTBT, FIG. 1) was capable of chemosensitizing C. albicans strains to fluconazole (Cernicka et al., “Chemosensitisation of drug-resistant and drug-sensitive yeast cells to antifungals.” Int. J. Antimicrobial Agents 2007, 29(2):170-8). Against fluconazole-susceptible C. albicans strain 90028 and fluconazole-resistant C. albicans strain Gu5, CTBT was effective with a minimum inhibitory concentration (MIC) value of 2.4 μM when combined with fluconazole. In the absence of fluconazole, CTBT demonstrated no activity against C. albicans strain 90028 but did inhibit growth of C. albicans strain Gu5 at concentrations greater than 2.4 μM. The anti-arrhythmic drug amiodarone was recently demonstrated to act synergistically with fluconazole in C. albicans with MIC values ranging between 1.6 μM to 18.8 μM (Gamarra et al., “Mechanism of the synergistic effect of amiodarone and fluconazole in C. albicans. Antimicrob Agents.” Chemother. 2010, 54(5):1753-61). Plagiochin E, a natural product isolated from liverwort, increased yeast susceptibility to fluconazole at 2.4 μM (Guo et al., “Plagiochin E, a botanic-derived phenolic compound, reverses fungal resistance to fluconazole relating to the efflux pump.” J. Appl. Microbio. 2008, 104(3):831-38). These agents have been reported to show single-agent antifungal activity. For example, amiodarone shows an MIC50 value of 3.1 μM (Courchesne, “Characterization of a novel, broad-based fungicidal activity for the antiarrhythmic drug amiodarone.” J. Pharmacol. Exp. Ther. 2002, 300:195-99;), and plagiochin E shows an IC50 value of 3.8 μM (Guo et al., “Plagiochin E, a botanic-derived phenolic compound, reverses fungal resistance to fluconazole relating to the efflux pump.” J. Appl. Microbio. 2008, 104(3):831-38).
There remains a need for new classes of antifungal agents or chemosensitizers that increase the effect of existing antifungals agents.
The most potent compounds currently known are several HDAC inhibitors previously reported by Mai et al. (“Discovery of uracil-based histone deacetylase inhibitors able to reduce acquired antifungal resistance and trailing growth in C. albicans.” Bioorg. Med. Chem. Lett. 2007, 17(5):1221-25). As depicted in FIG. 2, compounds 4 and 5 are uracil-derived hydroxamic acids that exhibited MIC values ranging from 1.2 μM to 1.4 μM when combined with fluconazole. When tested independently, neither compound demonstrated activity against C. albicans at concentrations up to 368 μM. When compounds 4 and 5 were evaluated in a biochemical binding assay with murine histone deacetylase 1 (HDAC1), their IC50 values were measured at 37 nM and 51 nM, respectively (Mai et al., “Discovery of uracil-based histone deacetylase inhibitors able to reduce acquired antifungal resistance and trailing growth in C. albicans.” Bioorg. Med. Chem. Lett. 2007, 17(5):1221-25). This finding suggests that these HDAC inhibitors would not be particularly selective for fungal protein targets and diminishes their potential as fungal-selective chemosensitizers. At the present time, neither compound 4 nor 5 has been registered with Molecular Libraries Small Molecule Repository (MLSMR) and was not available for evaluation.